Comparative Study of Structural Changes in NH3BH3, LiNH2BH3, and KNH2BH3 During Dehydrogenation Process

The thermal decomposition pathways of ammonia borane (AB, NH3BH3), lithium amidoborane (LiAB, LiNH2BH3), and potassium amidoborane (KAB, KNH2BH3) have been investigated in detail by using solid state B-11 MAS NMR spectroscopy. During the first decomposition process of AB, the complex structural entities were observed, which have been attributed to polyaminoborane (PAB, (NH2BH2)(n)). On the other hand, polyiminoborane (PIB, (NHBH)(n)) formed in the second process showed a single 3-fold coordination B-111 signal above 125 degrees C, suggesting a [-B=N-](n) network structure. LiAB and KAB did not show the PAB-like structure, and the PIB-like structure was directly formed by the hydrogen desorption. Quantitative analyses of the B-11 NMR spectra suggested that the general route of the thermal decomposition of alkali metal amidoboranes (LiAB, NaAB, and KAB) is similar. Also, K-39 MAS NMR spectra of KAB and its decomposition products indicated the formation of KR and an amorphous K-N-B-H phase during the hydrogen release.